Tunic abnormalities of the urochordate Halocynthia pyriformis

This paper reports lesions of the tunicate Halocynthia pyriformis, which either grossly resembled supernumerary siphons or appeared as elongate protrusions on the surface of the tunic. The lesions were present on freshly collected animals; they did not develop during maintenance in the laboratory. The siphon-like structures were doughnut-shaped thickenings of noncellular tunic material around a vertical core of necrotic debris; the elongate protrusions were also tubular thickenings of tunic material surrounding a necrotic core. Both types of lesions were extensively infiltrated with blood cells and possibly exfoliated epidermal cells proximal to the necrotic cores, suggestive of an inflammatory response. Bacteria were seen within the debris of the elongate protrusions. These lesions were interpreted as injury and repair responses of the tunic rather than developmental anomalies or neoplasia. Their etiology could not be precisely determined. The bacteria associated with the intracystic debris of the elongate lesions could have been causative or only secondary invaders. Possibly the postulated injury resulted from a chance abrasion, puncture, contact with toxic material, or from a parasite. An intimate association of numerous hydrozoids around the base of the animal and a few on the upper portion of the tunic suggested a plausible explanation of at least some of these lesions. Occasionally, one observed the hydrorhiza of a hydrozoan embedded in the tunic itself; living hydranths were seen extending from these structures. The tunic was thickened in the region of the hydrorhiza, and histological examination of these thickenings showed details quite similar to those reported above. There was a core of necrotic debris and considerable cellular infiltration of the tunic. Probably these lesions represent the result of a primitive cellular immune response of the tunicate against the hydrozoans, or against secondary bacterial infections associated with the presence of the hydrozoans in the tunic.